perfSONAR User Q&A and Other Discussion

[Matthew J Zekauskas <>]

The convention in higher ed has been to use 9000 flat for the edge MTU when configuring "Jumbo frames".  See this fairly old statement for some background: <http://noc.net.internet2.edu/i2network/maps-documentation/policy-statements.html#Jumbo Frames>  MTU is the total packet length with headers.    The 8972 value is payload without headers to reach 9000 on the wire, so you don't want to set interface MTU to 8972 or the packets you can receive would max out at 8972 instead of 9000.

More pointers including some of the downsides and things to watch out for are in this fasterdata article: <http://fasterdata.es.net/network-tuning/mtu-issues/>

The switches (and everything else in that particular broadcast domain) need to also be at 9000 to have seamless operation.   Switches and intermediate nodes/routers may need to be higher -- say the 9000 byte frame is packaged in a 802.1q VLAN, you need to add an extra 8 bytes for VLAN headers.   I believe we just set link MTUs to be as high as they can be, and the particular (sub)interface MTU to be 9000.  (Some of this is in the Internet2 statement.)

If switch (or router) ports are not configured for large frames, then they will report "giant" errors if they receive a jumbo frame.

--Matt

On 8/11/16 6:11 PM, Jennewein, Douglas M wrote:

Hi, folks

This article got us thinking about the MTU configuration on our PerfSONAR nodes.

https://www.mylesgray.com/hardware/test-jumbo-frames-working/ 

At the OS level, should our nodes use 8972 instead of 9000?

Can this cause “giant” errors on the switch ports the PerfSonar nodes are connected to?

-- 
Doug Jennewein

Director of Research Computing

The University of South Dakota